1. Field of the Invention
The present invention relates to a power module substrate for use in a power module which dissipates heat, a method of producing the same, and a semiconductor device including the substrate, and more particularly to a power module substrate which is so structured as to be joined directly to a water-cooling type heat sink by means of male screws, a method of producing the same, and a semiconductor device including the substrate.
2. Discussion of the Background
As a power module substrate of the above type, as shown in FIG. 22, known is a substrate in which a ceramic substrate 1 is made of AIN, and to the opposite sides of the ceramic substrate 1, first and second copper plates 2 and 3 are laminated and bonded, and an Ni plating is formed on the upper side of a heat sink 4 made of Cu, and further, the heat sink 4 is laminated and bonded to the second copper sheet 3 through a solder 6. In the case of a semiconductor device having a semiconductor element 7 mounted onto this substrate, the heating quantity is relatively large. Accordingly, the semiconductor device is joined to a water-cooling type heat sink 8 which transfers the heat outside forcedly by circulating cooling water 8a inside thereof. The attachment of the power module substrate to the water-cooling type heat sink 8 is carried out by forming attachment holes 4a in the heat sink 4, and pushing male screws 9 through the attachment holes 4a and screwing the male screws in female screws 8b formed in the water-cooling type heat sink 8. In the semiconductor device joined as described above, heat emitted from the semiconductor element and so forth is dissipated outside from the water-cooling type heat sink 8 through the first copper sheet 2, the ceramic substrate 1, the second copper sheet 3, the solder 6, and the heat sink 4.
However, in the above-described conventional semiconductor device, the heat transfer route from the semiconductor element 7 or the like to the water-cooling type heat sink 8 is relatively long. In particular, inconveniently, heat from the semiconductor element 7 can not be efficiently transferred to the water-cooling type heat sink 8, since the second copper sheet 3 is laminated and bonded to the water-cooling type heat sink 8 through the solder 6 having a low thermal conductivity. To solve this problem, it may be proposed that attachment holes 1a are formed directly in the ceramic substrate 1 without the heat sink being provided, the male screws 9 are inserted through the attachment holes 1a, and screwed in the female screw 8b formed in the water-cooling type heat sink 8, as shown in FIG. 21, so that the heat transfer route from the semiconductor element to the water-cooling type heat sink 8 is shortened.
However, there is the problem that it is very difficult to form the attachment holes 1a after the ceramic substrate is fired, since the substrate 1 after firing is rigid and brittle. Further, as regards forming the attachment holes 1a before firing, and then, firing the ceramic substrate 1, there is the problem that the pitch of the attachment holes 1a can not be exactly produced due to the shrinkage at firing. Even if the attachment holes 1a can be accurately formed, there is the danger that the ceramic substrate 1, which is brittle, may be cracked, caused by the tightening force of the male screws 9 generated when the ceramic substrate 1 is joined to the water-cooling type heat sink 8.
Accordingly, it is an object of the present invention to provide a power module substrate in which the heat transfer route from a semiconductor element to a water-cooling type heat sink is shortened, and thereby, heat from the semiconductor element can be effectively dissipated without the ceramic substrate being damaged, a method of producing the same, and a semiconductor device including the substrate.
According to the present invention, as shown in FIGS. 1 and 6, there is provided a power module substrate which comprises a ceramic substrate 11 having a circuit pattern 17 formed on the surface thereof, and a metal frame 12 provided on the periphery of the ceramic substrate 11 and so structured that the ceramic substrate 11 can be joined to a water-cooling type heat sink 27.
In this power module substrate, the ceramic substrate 11 is joined to the water-cooling type heat sink 27 through the metal frame 12. Therefore, no external force is applied directly to the ceramic substrate 11, and breaking of the ceramic substrate 11, caused by the joining, is prevented. Heat from the semiconductor device mounted onto the circuit pattern 17 can be effectively transferred to the water-cooling type heat sink 27 and dissipated.
Preferably, the ceramic substrate 11 is formed with AIN, Si3N4, or Al2O3. When AIN is used as the ceramic substrate 11, the thermal conductivity and the heat resistance are enhanced. The use of Si3N4 improves the strength and the heat resistance. With the use of Al2O3, the heat resistance is enhanced.
Preferably, in the above power module substrate, the metal frame 12 has a thickness equal to that of the ceramic substrate 11 or the ceramic substrate 11 having the circuit pattern 17, and is provided with plural perforations 12a formed so as to sandwich the ceramic substrate 11, and metal thin sheets 13 having through-holes 13a in communication with the corresponding perforations 12a, and containing contacting portions 13b having the undersides thereof contacted to at least a part of the circumferential surface of the ceramic substrate 11 or the circuit pattern 17 are disposed on the surface of the metal frame 12, whereby the ceramic substrate 11 having the circuit pattern 17 formed thereon and contacted to the undersides of the contacting portions 13b can be joined into the water-cooling type heat sink 27 by inserting male screws 26 through the through-holes 13a and the perforations 12a, and screwing the male screws 26 in female screws 27a formed in the water-cooling type heat sink 27 or further inserting the male screws 26 through attachment holes 27c formed so as to perforate the water-cooling type heat sink 27 and screwing the male screws in nuts 31.
As described above, the perforations 12a and the through-holes 13a are formed in the metal frame 12 and the metal thin sheets 13 bonded to the surface of the metal frame 12, correspondingly. Accordingly, when the male screws 26 are inserted through the through-holes 13a and the perforations 12a, and screwed in the female screws 27a (FIG. 2C) formed in the water-cooling type heat sink 27, or further inserted through the attachment holes 27c formed so as to perforate the water-cooling type heat sink 27 and screwed in nuts 31 (FIG. 6), the tightening force of the male screws 26 is not applied directly to the ceramic substrate 11, preventing the breaking of the ceramic substrate 11, which may be caused by the tightening force of the male screws 26. Heat from a semiconductor element mounted onto the circuit pattern 17 can be effectively transferred to the water-cooling type heat sink 27 and dissipated.
In the case that the metal frame 12 and the metal thin sheets 13, disposed on the surface of the metal frame 12, are made of a material which can be machined relatively easily as compared with the ceramic substrate 11, and the through-holes 13a and the perforations 12a are formed in the metal thin sheets 13 and the metal frame 12, correspondingly, so as to perforate them, attachment holes can be formed in the power module substrate easily and at a high precision attachment pitch.
Also preferably, as shown in FIG. 8, a metal frame 62 has a thickness greater than that of the ceramic substrate 11 or the ceramic substrate 11 having the circuit pattern 17, and is provided with plural perforations 62a formed so as to sandwich the ceramic substrate 11, metal thin sheets 63 having through-holes 63a in communication with the corresponding perforations 62a, containing opposing portions 63b having the undersides thereof opposed to at least a part of the circumferential surface of the ceramic substrate 11 or the circuit pattern 17 is disposed on the surface of the metal frame 62, and elastic pieces 64 each having a thickness equal to the difference between the ceramic substrate 11 or the ceramic substrate 11 having the circuit pattern 17 and the metal frame 62 or being slightly larger than the difference are interposed between the surface of the ceramic substrate 11 or the circuit pattern 17 and the opposing portions 63b, respectively, whereby the ceramic substrate 11 having the circuit pattern 17 formed thereon and contacted to the undersides of the opposing portions 63b through the elastic pieces 64 can be joined to the water-cooling type heat sink 27 by inserting male screws through the through-holes 63a and the perforations 62a and screwing the male screws 26 in the female screws 27a formed in the water-cooling type heat sink 27, or further inserting the male screws 26 through the attachment holes 27c formed in the water-cooling type heat sink 27 and screwing the male screws 26 in the nuts 31, respectively.
In the above power module substrate, breaking of the ceramic substrate 11, caused by the tightening force of the male screws 26, is prevented. Heat from the semiconductor element 23 mounted onto the circuit pattern 17 can be effectively transferred to the water-cooling type heat sink 27, and moreover, the elastic pieces 64 interposed between the surface of the ceramic substrate 11 or the circuit pattern 17 and the opposing portions 63b, respectively, absorb the attachment error between the ceramic substrate 11 and the water-cooling type heat sink 27, caused by the expansion or shrinkage. Thus, breaking of the ceramic substrate 11, caused by the change of temperature, is prevented.
More preferably, in the power module substrate, both of the upper side and the underside of each elastic piece 64 are bonded to the surface of the ceramic substrate 11 or the circuit pattern 17 and the surface of the corresponding opposing portion 63b through a heat resistant adhesive 66, respectively.
In this power module substrate, since the elastic piece 64 is interposed and bonded between the surface of the ceramic substrate 11 or the circuit pattern 17 and the corresponding opposing portion 63b, the substrate is prevented from being displaced in its use environment, which may be caused by vibration or the like, so that the ceramic substrate 11, which has the circuit pattern 17 in contact to the undersides of the opposing portions 63b through the elastic pieces 64, can be effectively bonded to the water-cooling type heat sink 27.
More preferably, in the power module substrate of the present invention, each of the elastic pieces 64 has a rectangular cross-section, and the ratio Y/X is at least 0.08 in which X represents the width of the cross-section and Y the thickness of the elastic piece 64.
Preferably, in the power module substrate of the present invention, the ceramic substrate 11 has a metal foil 11a bonded to the back thereof, and the bearing pressure P of the metal foil 11a through which the ceramic substrate 11 is bonded to the water-cooling type heat sink 27, against the water-cooling type heat sink 27, and the coefficient xcexc of friction between the metal foil 11a and the water-cooling type heat sink 27 has a relationship expressed as the formula of xcexcPxe2x89xa610 (MPa).
Accordingly, the displacement in the horizontal direction of the ceramic substrate 11, caused by the thermal expansion, is enabled, and breaking of the ceramic substrate 11 is prevented.
Preferably, in the power module substrate of the present invention, as shown in FIGS. 9 and 13, the metal frame 72 is provided on at least a part of the periphery of the ceramic substrate 11, has a thickness equal to or slightly smaller than that of the ceramic substrate 11, and has plural perforations 72a formed so as to sandwich the ceramic substrate 11, a first metal thin sheet 73 having first through-holes 73a in communication with the corresponding perforations 72a, having a circuit pattern 77 formed in the part of the first metal thin sheet 73 opposed to the ceramic substrate 11 is bonded to the surface of the ceramic substrate 11 and that of the metal frame 72 through a soldering material 76, and a second metal thin sheet 74 having second through-holes 74a in communication with the perforations 72a and the first through-holes 73a, respectively, and opposed to the water-cooling type heat sink 27 is bonded to the back of the ceramic substrate 11 and that of the metal frame 72 through the soldering material 76, whereby the ceramic substrate 11 can be joined to the water-cooling type heat sink 27 by inserting the male screws 26 through the first through-holes 73a, the perforations 72a, and the second through-holes 74a, screwing the male screws 26 in the female screws 27a formed in the water-cooling type heat sink 27 or further inserting the male screws 26 through the attachment holes 27c formed in the water-cooling type heat sink 27 and screwing the male screws 26 in the nuts 31.
In the above-described power module substrate, the first through-holes 73a, the perforations 72a, and the second through-holes 74a are formed in the metal frame 72 integrated with the ceramic substrate 11, and the first and second metal thin sheets 73 and 74 bonded to the front and the back of the metal frame 72, correspondingly. Accordingly, when the male screws 26 are inserted through the first through-holes 73a, the perforations 72a, and the second through-holes 74a, screwed in the female screws 27a formed in the water-cooling type heat sink 27 or further inserted through the attachment holes 27c formed in the water-cooling type heat sink 27 and screwed in the nuts 31, no tightening force of the male screws 26 is applied directly to the ceramic substrate 11, which prevents breaking of the ceramic substrate 11, caused by the tightening force of the male screws 26. Heat from the semiconductor element 23 mounted onto the circuit pattern 77 can be effectively transferred to the water-cooling type heat sink 27.
Also more preferably, in the power module substrate of the present invention, as shown in FIGS. 14 and 19, the metal frame 112 is secured to at least a part of the periphery of the ceramic substrate 111, and has plural perforations 112a formed so as to sandwich the ceramic substrate 111, collars 116 each comprising a cylindrical portion 116a having a through-hole 116c and a flange 116b in contact to the upper side of the metal frame 112, which are formed integrally with each other, are floating-inserted through the perforations 112a, respectively, elastic pieces 117 are interposed between the flange portions 116b and the upper side of the metal frame 112, respectively, and whereby the ceramic substrate 111 having the metal frame 112 secured thereto can be joined to the water-cooling type heat sink 27 by inserting the male screws 26 through the perforations 116c, screwing the male screws 26 in the female screws 27a formed in the water-cooling type heat sink 27 or further inserting the male screws 26 through the attachment holes 27c formed in the water-cooling type heat sink 27 and screwing the male screws 26 in the nuts 31.
More preferably, as shown in FIG. 16, in the power module substrate of the present invention, the metal frame 112 is secured to at least a part of the periphery of the ceramic substrate 111, and has the plural insertion holes 112a formed so as to sandwich the ceramic substrate 111, washers 118 having communication holes 118a in communication with the corresponding insertion holes 112b are disposed on the upper side of the metal frame 112, and the elastic pieces 117 are interposed between the washers 118 and the upper side of the metal frame 112, respectively, whereby the ceramic substrate 111 having the metal frame 112 secured thereto can be joined to the water-cooling type heat sink 27 by floating-inserting the male screws 26, inserted through the communication holes 118a, through the insertion holes 112b, screwing the male screws 26 in the female screws 27a formed in the water-cooling type heat sink 27 or further inserting the male screws 26 through the attachment holes 27c formed in the water-cooling type heat sink 27 and screwing the male screws 26 in the nuts 31.
As seen in the above description, the perforations 112a and the insertion holes 112b, as they are formed in the metal frame 112 secured to the ceramic substrate 111, can be formed easily and at a high precision pitch as compared with the case where the attachment holes 1a are formed directly in the ceramic substrate 1 as shown in FIG. 21.
Further, when the ceramic substrate 11 is joined to the water-cooling type heat sink 27 with the male screws 26 and by use of the perforations 112a and the insertion holes 112b, the tightening force of the male screws 26 is not applied directly to the ceramic substrate 111, preventing the breaking of the ceramic substrate 111, caused by the tightening force of the male screws 26. Heat from the semiconductor element 23 mounted onto the circuit pattern 111a can be effectively transferred to the water-cooling type heat sink 27.
Moreover, the elastic pieces 117 interposed between the collars 116 or the washers 118 and the upper side of the metal frame 112, respectively, absorb the attachment error between the ceramic substrate 111 and the water-cooling type heat sink 27, caused by the expansion or shrinkage, which is contributed by the elasticity. Thus, breaking of the ceramic substrate 111, caused by the change of temperature, is prevented.
More preferably, in the power module substrate of the present invention, the metal frame 112 is secured to the ceramic substrate 111 at least partially by soldering or welding.
By soldering or welding as described above, the metal frame 112 can be secured to the ceramic substrate 111 easily and steadily.
Still more preferably, in the power module substrate of the present invention, the ceramic substrate 11 has a thickness of from 0.2 mm to 3.5 mm.
If the thickness of the ceramic substrate 11 is less than 0.2 mm, the ceramic substrate 11, which is joined to the water-cooling type heat sink 27 through the metal frame by screwing of the male screws 26, may be broken, caused by the tightening force of the male screws 26. If the thickness of the ceramic substrate 11 exceeds 3.5 mm, the mechanical strength of the ceramic substrate 11 itself is so high that it can be joined directly to the water-cooling type heat sink 27 with the male screws 26.
According to the present invention, as shown in FIG. 9, there is provided a method of producing the power module substrate which comprises the steps of providing the metal frame 72 having a thickness equal to or slightly smaller than that of the ceramic substrate 11 on at least a part of the periphery of the ceramic substrate 1, bonding the first and second metal thin sheets 73 and 74 to the surface of the ceramic substrate 11 and the metal frame 72 through the soldering material 76 whereby the ceramic substrate 11 is integrated with the metal frame 72, forming the circuit pattern 77 in the portion of the first metal thin sheet 73 which corresponds to the ceramic substrate 11, and forming the first through-holes 73a, the perforations 72a, and the second through-holes 74a in the first metal thin sheet 73, the metal frame 72, and the second metal thin sheet 74, correspondingly, so as to perforate them.
According to the method of producing the power module substrate, the metal frame 72 integrated with the ceramic substrate 1, and the first and second metal thin sheets 73 and 74 bonded to the front and the back of the metal frame 72, respectively, can be machined easily as compared with the ceramic substrate 11. The first through-holes 73a, the perforations 72a, and the second through-holes 74a are formed so as to perforate the first metal thin sheet 73, the metal frame 72, ad the second metal thin sheet 74, correspondingly, and therefore, the attachment holes can be formed easily at a high precision attachment pitch in the power module substrate.
According to the present invention, there is provided a semiconductor device, as shown in FIG. 2, in which the semiconductor element 23 is mounted on the circuit pattern 17 of the power module substrate 21 of the present invention, a frame piece 25 having terminals 24 provided on the inner periphery thereof is bonded to the surface of the power module substrate 21 so as to surround the semiconductor element 23, the terminals 24 and the semiconductor element 23 are connected to each other, and an insulating gel 29 is filled, a lid plate 25a is bonded to the upper side of the frame piece 25, the male screws 26 are inserted through the through-holes 13a of the metal thin sheet 13 and the perforations 12a of the metal frame 12 in the power module substrate 21, and the power module substrate 21 is joined directly to the water-cooling type heat sink 27 by screwing the male screws 26 in the female screws 27a formed in the water-cooling type heat sink 27 or further inserting through the attachment holes (not shown in FIG. 2) formed so as to perforate the water-cooling type heat sink 27, and screwing the male screws 26 in the nuts.
In this semiconductor device, the heat transfer route from the semiconductor element 23 mounted onto the circuit pattern 17 of the power module substrate 21 joined directly to the water-cooling type heat sink 27 is shorter than the conventional one as shown in FIG. 22, so that heat from the semiconductor element 23 can be more effectively transferred to the water-cooling type heat sink 27 and dissipated outside, as compared with the conventional one.
Further, as shown in FIG. 7, according to the present invention, there is provided a semiconductor device in which the water cooling heat sink 27 comprises a heat sink body 27d having a water passage 27b, and a heat sink lid 27e capable of sealing the water passage 27b, the semiconductor element 23 is mounted to the circuit pattern 17 of the power module substrate 21 of the present invention, the male screws 26 are inserted through the through-holes 13a of the metal thin sheet 13 and the perforations 12a of the metal frame 12 of the power module substrate 21, the power module substrate 21 is joined directly to the heat sink lid 27e by screwing the male screws 26 in the female screws 27f formed in the heat sink lid 27e, a frame piece 25 having terminals 24 provided on the inner periphery thereof is bonded to the surface of the heat sink lid 27e so as to surround the power module substrate 21, the terminals 24 are connected to the semiconductor element 23, and the insulting gel 29 is filled, a lid plate 25a is bonded to the upper side of the frame piece 25, and the heat sink lid 27e is screwed to the heat sink body 27d. 
In this semiconductor device, the heat transfer route from the semiconductor element 23 to the water-cooling type heat sink 27 is also shorter than the conventional one, so that heat from the semiconductor element 23 can be more effectively transferred to the water-cooling type heat sink 27 and dissipated outside. In particular, in this semiconductor device, the power module substrate 21 and so forth are previously mounted onto the heat sink lid 27e. Accordingly, the semiconductor device can be obtained by simple working, that is, only by screwing the heat sink lid 27e to the heat sink body 27d. 
Still further, as shown in FIG. 10, according to the present invention, there is provided a semiconductor device in which the semiconductor element 23 is mounted to the circuit pattern 77 of the power module substrate 81 of the present invention, the frame piece 25 having the terminals 24 provided on the inner periphery thereof is bonded to the surface of the power module substrate 81 so as to surround the semiconductor element 23, the terminals 24 are connected to the semiconductor element 23, and the insulating gel 29 is filled, the lid plate 25a is bonded to the upper side of the frame piece 25, and the power module substrate 81 is bonded directly to the water-cooling type heat sink 27 with the male screws 26.
In this semiconductor device, the heat transfer route from the semiconductor element 23 mounted onto the circuit pattern 77 of the power module substrate of the present invention, joined directly to the water-cooling type heat sink 27 to the water-cooling type heat sink 27 is shorter than the transfer route of the conventional semiconductor device as shown in FIG. 22. Heat from the semiconductor element 23 can be more effectively dissipated outside through the water-cooling type heat sink 27 as compared with the conventional one.
Further, as shown in FIGS. 14, 16, 19, and 20, according to the present invention, there is provided a semiconductor device in which the semiconductor element 23 is mounted to the circuit pattern 111a of the power module substrate 110, 120 of the present invention, the frame piece 25 having the terminals 24 provided on the inner periphery thereof is bonded to the surface of the power module substrate 110, 120 so as to surround the semiconductor element 23 (FIG. 20), the terminals 24 are connected to the semiconductor element 23, and the insulating gel 29 is filled, the lid plate 25a is bonded to the upper side of the frame piece 25, and the male screws 26 are inserted through the through-holes 116c (FIG. 14) of the collars 116 according to the present invention and the communication holes 118a of the washers 118 according to the present invention and the insertion holes 112b of the metal frame 112 (FIG. 16) according to the present invention, and the power module substrate 110, 120 is joined directly to the water-cooling type heat sink 27 by screwing the male screws 26 in the female screws 27a (FIG. 20) formed in the water-cooling type heat sink 27 or further inserting the male screws 26 through the attachment holes 27c formed so as to perforate the water-cooling type heat sink 27, and screwing the male screws 26 in the nuts 31 (FIG. 19).
In this semiconductor device, the heat transfer route form the semiconductor device 23 mounted onto the circuit pattern 11a of the power module substrate 110, 120 joined directly to the water-cooling type heat sink 27, to the water-cooling type heat sink 27 is shorter than the conventional one as shown in FIG. 22, so that heat from the semiconductor element 23 can be more effectively transferred to the water-cooling type heat sink 27 and dissipated outside, as compared with the conventional one.